This invention generally relates to voice mail systems and more particularly to system improvements in detection of messages and valid DTMF signals or tones and in cancellation of a previously recorded audio message from a message to be recorded.
As explained in depth in other patents such as U.S. Pat. No. 4,371,752 to Matthews and U.S. Pat. No. 4,747,126 to Hood et al. owned by the present assignee, a voice mail system records and plays back telephone messages intended for one of a plurality of system users. The caller is typically greeted and instructed by prompt signals, to which he responds by entering touch tone (DTMF) commands. The caller can thereby record a message, review it, or perform other system operations. The user similarly can use DTMF commands for system control, such as to retrieve or delete messages for him.
A recurring problem in voice mail systems is triggering of the system by noncommand or invalid DTMF signals. These signals can occur both in a message 1 received for recording, as a result of a caller's voice, and in the playback of a previously recorded message, as a result of voice or a DTMF command entered by the caller. A tone detector within the system is adapted to respond to DTMF commands during both record and playback and may inadvertently respond to an invalid signal.
Prior attempts to solve the playback problem have only been partially effective. The method described in U.S. Pat. No. 4,747,126 limits recorded tones to a duration shorter than is required for the tone detector on playback to respond. But these and other methods do not effectively handle DTMF signals produced on playback by the caller's voice, a voice prompt in the system, signal noise, etc., that contain the same tones as DTMF signals.
Another problem present in prior voice mail systems is line interference with the caller-generated DTMF commands. Different telephone systems have different impedances in their central office lines which affect the transmission and echoing of audio signals to a voice mail system. Matching the impedance of all possible systems to the voice mail system is not practical. Prior voice mail systems have generally incorporated a single characteristic impedance that will work passively with whatever telephone system is ultimately connected to the mail system. However, telephone lines that differ in impedance from the characteristic impedance can interfere with valid DTMF tone commands generated by the caller, causing the voice mail system to ignore the command in the presence of a voice prompt or other previously recorded audio message. For example, the system may not hang up because a DTMF command indicating the end of a message was obscured. Poor matching of the telephone system and voice mail system impedance can also affect the ability of the voice mail system to cancel a previously recorded message from a message to be recorded.
Yet another problem found in prior voice mail systems is their inability to hang up once a caller has finished his message. Most systems ask the caller for a DTMF command to indicate the message has ended and that the system may hang up. Many callers fail to enter that command. Moreover, the command may not successfully pass through the interference described above. Other systems include an audio detection circuit as a backup detection, causing the system to hang up after a predetermined length of silence. But noise or central office generated call progress tones can often produce sufficient audio energy to fool the audio detection circuit. A fail-safe method is sometimes employed, such as terminating the call after a predetermined time unless the caller takes further action such as pressing a telephone key. Fail-safe methods are effective but annoying to a caller and inefficient because it wastes limited system storage.
Prior systems also include means for indicating to each user at his telephone the status of the system and whether he has a message waiting. A drawback of such indicators is that they are limited to the telephone location and do not reach the person who is frequently out of his office in a laboratory or common work area.